Use the target-lexicon crate.

This switches from a custom list of architectures to use the
target-lexicon crate.

 - "set is_64bit=1; isa x86" is replaced with "target x86_64", and
   similar for other architectures, and the `is_64bit` flag is removed
   entirely.

 - The `is_compressed` flag is removed too; it's no longer being used to
   control REX prefixes on x86-64, ARM and Thumb are separate
   architectures in target-lexicon, and we can figure out how to
   select RISC-V compressed encodings when we're ready.

lib/codegen/src/lib.rs

@@ -52,6 +52,11 @@ extern crate alloc;
 extern crate failure;
 #[macro_use]
 extern crate failure_derive;
+#[cfg(test)]
+#[macro_use]
+extern crate target_lexicon;
+#[cfg(not(test))]
+extern crate target_lexicon;
 
 pub use context::Context;
 pub use legalizer::legalize_function;